1. Field of the Invention
The present invention relates to information signal storage, more particularly to information storage in photorefractive crystals.
2. Discussion of Prior Art
It is well known that light impinging upon photorefractive crystals serving as a record-medium such as bismuth silicon oxide (BSO), when activated by a reference beam, causes interference of the two monochromatic light beams within the crystal. The light interference results in storage of data carried by the light beams as a volume hologram in a portion of the record medium. Readout of the stored hologram is achieved by shining a reference light beam into the record medium for causing the reference beam to be modulated by the stored hologram as it passes through the record medium. The modulated reference beam leaves the record medium along an output light path. It is known to superpose holograms in a thick photorefractive medium, such as BSO.
The L. d'Auria et al. report in the periodical APPLIED OPTICS in an article entitled "Experimental Holographic Read-Write Memory Using 3-D Storage", Vol. 13 No. 4, April 1974, in pages 808-818 advises that holograms can be superposed in a photorefractive record medium by rotating the reference beam. The rotation of the reference beam causes it to have different angles of incidence on the input surface of the thick recording medium. Because of different angles of incidence, the modulated light beam carrying the information to be stored creates a plurality of holograms superposed in the same volume. Each of the holograms are readable by a readout reference beam aligned at the angle used for recording the respective superposed holograms. Such recording has required that the intensity of the pixels in the hologram be modulated to carry information. The intensity modulated pixels are subject to noise generated by the optical path through which the pixels travel.
It is also well known that a laser monochromatic light beam in addition to carrying information based upon light intensity modulation, can carry information at different angles of rotation of the linear polarization of the light beam with respect to a reference linear light polarization. Such polarization rotation results in P and S components, which are orthogonally disposed in the light beam. An example of such modulation is the readback light beam reflected from or transmitted through a magneto optic record medium. The readback optics and electrical circuits for a magnetooptic record medium process the P and S components of the modulated reflected light beam for generating modulated output electrical signals. The rotational modulation of light polarization carry information in its P and S components. Many optical systems process light beams through polarization sensitive beam splitters and other elements. An example of such light beam processing is shown in U.S. Pat. Nos. 3,544,189 and 3,622,220. Accordingly, it is well known to separate the P and S components and to rotate the linear polarization of light beams for achieving various optical separation functions.
U.S. Pat. No. 3,825,316 discloses a holographic system in which the polarization of the light is switched in either of two directions for detection. According to this patent, a laser produces a beam of polarized coherent light which is transmitted through a polarization switching device that adjusts, in response to an input signal, the orientation of the plane of polarization of the transmitted light in either one of two mutually orthogonal states. The light transmitted by the polarization switching device then impinges on a transparent electro-optic crystal material whose index of refraction along its C axis varies in accordance with the interference fringes of a pre-established holographic pattern. In one embodiment disclosed in this patent, the optical system includes an electro-optic beam deflector to provide a strong-polarization-dependent digital laser beam deflection system such that the beam is deflected in a different predetermined direction depending upon the polarization state of the beam, i.e. beam splitting. In another embodiment shown in this patent, the optical system includes a complex optical component to provide light deflection apparatus for selectively deflecting a beam of light from a source to one of a plurality of target positions.
All of the above references, with regard to holograms, show an intensity modulated beam for carrying information; it is also known that a single light beam carries information by rotational modulation of the linear light polarization. It is desired to provide a more efficient and noise free holographic signal storage system wherein the information is carried by rotational modulation of linear polarization of laser light beams.